The present invention relates to a carrier detector utilized in facsimile transceivers for transmitting the contents of a document to remote locations using standard telephone transmission facilities.
The functions of a facsimile transceiver system is to scan documents at a transmit station and to develop an information signal representative of the contents of the document, the information signal varying in a voltage range between a first potential corresponding to a white document area and a second potential corresponding to a dark document area. This information signal is then modulated into a form suitable for transmission over standard telephone transmission lines. The preferred form of modulation for such baseband signals is to frequency modulate them on a carrier signal into the audio range between arbitrary frequencies f1 and f2 transmittable by ordinary telephone circuitry, generally in the range of 1500 Hz to 2500 Hz.
The frequency modulated facsimile signal is then coupled into standard telephone transmission lines and taken therefrom again at the receiving station to the same standard handsets that are used for regular voice transmission, so that no special jacks or other electrical hookups are required. At the receiving station the above-mentioned frequency modulated facsimile signal is demodulated to recover the information signal which then operates a recorder or printing device. The printing device then reproduces the contents of the document originally scanned at the transmit station.
In prior art facsimile systems, in order to start up the recorder at the receiver unit prior to the actual transmission containing signals, a start control signal is transmitted by the transmit unit consisting of a relatively long burst of one frequency f3 (which may correspond to a dark document area) followed by a short burst of another frequency f1 (which may correspond to a white document area). This control signal is repeated for a preselected time period. At the receiver the above-noted start control signal is demodulated into a varying d.c. voltage signal similar to that illustrated in the left-hand portion of FIG. 2A, which signal is applied to a detector circuit of the invention in order to turn on the recorder. The detector therefore is designed to be particularly sensitive to a wave-shape of the type shown in FIG. 2A and correspondingly insensitive to noise or random signals at its input.
The print device or recorder at the receive station is likewise disabled or shut down after the completion of a transmitted document by a stop control signal generated at the transmit unit after the original document has been scanned. In the prior art machines, this stop control signal usually consists of a relatively long period of a continuous frequency f4 which is outside of the range f1 - f3 mentioned above and which is demodulated at the receiver and presented to the detector circuit of the invention as a constant d.c. voltage similar to the one illustratively shown in the right-hand portion of FIG. 2A.
In addition, it is desirable to have a preselected time delay associated with both the start and stop operations. Specifically, start up of the recorder may be designed to occur a preselected time after receipt of the start control signal and shut down occur approximately the same preselected time delay after receipt of the stop control signal. For this reason, the detector circuit of the invention comprises a circuit arrangement for enabling and disabling a facsimile receiver a preselected time after receipt of start and stop control signals, respectively, of the type shown in FIG. 2A. The detector circuit is also designed to be most sensitive to a start signal having the particular wave-shape shown in FIG. 2A.
U.S. Pat. No. 3,593,151, assigned to the assignee of the present invention, describes a detector for use in a facsimile receiver which monitors the FM input carrier signal and activates a print transducer motor when a 2200 Hz or greater carrier signal is detected. The motor, which rotates the transducer scanning drum, subsequently remains on as long as the carrier is at a frequency above 1400 Hz. The detector includes a transistor comparator which is biased initially to respond to a 2200 Hz carrier signal. When this carrier signal is received, the comparator generates an output signal which causes the printing transducer motor to be energized. Simultaneously, the bias applied to the input of the comparator is adjusted to a new value equal to the sum of the initial bais and a bias voltage obtained from the power supply energized by the initially received 2200 Hz carrier signal. The comparator is then biased so that a carrier signal of a frequency of 1400 Hz or greater will maintain the generation of the output signal.